The insulin-like growth factor 1 (IGF-1 receptor (IGF-1R) is an essential regulator of cell growth and transformation. IGF-1 and IGF-2, via the IGF-1R, are potent breast cancer cell mitogens that promote the tumorigenic potential of cancer cells. The objective of this proposal is to develop reagents that block IGF-1R signaling. The insulin-like growth factor binding proteins (IGFBPs) bind IGF-1 and IGF-2 with higher affinities than the IGF-1R and serve to both protect the IGFs from degradation and reduce their delivery to the IGF-1R. The hypothesis of the proposal is that blockade of IGF-1R activity can be accomplished by developing IGF antagonists based on the structure of the IGF binding domain on the IGFBPs. The goal of Aim 1 is to define the structure of the IGF-binding domain on rhIGFBP-2 using photoaffinity labeling and mass spectrometric analyses. To this end, IGF-1 derivatized with photactivatable groups within its IGFBP-binding domain will be synthesized. This will allow the precise identification of the sites of interaction between rhIGFBP-2 and IGF-1. Based on these analyses, deletion, truncation and site-directed mutants of IGFBP-2 will be generated and tested for IGF binding activity. Aim 2 will characterize the structure and function of a 15.8 kDa fragment of IGFBP-2. The goal of Aim 3 is to examine the mechanism responsible for IGFBPP-2 inhibition of IGF action. The goal of Aim 4 is to employ phage display to screen libraries for peptides having a high affinity for the IGFBP-binding domain on IGF-1 as an alternative means of designing IGF antagonists.